1. Field of the Invention
The invention relates to a piezoelectric actuator sheathed with a sleeve and bathed for instance by liquid media, which can be inserted into a piezoelectric injector for chronologically and quantitatively precise metering of fuel for an internal combustion engine. A piezoelectric injector of this kind substantially comprises a holder body and the piezoelectric actuator disposed in the holder body, the piezoelectric actuator having piezoelectric elements stacked one above the other and disposed in an actuator head and an actuator foot.
2. Description of the Prior Art
It is known per se that for constructing a piezoelectric actuator of the type described above, the piezoelectric elements can be inserted in such a way that by utilizing the so-called piezoelectric effect, a control of the needle stroke of a valve or the like can be accomplished. Piezoelectric layers of the piezoelectric elements are constructed from a material with a suitable crystalline structure, in such a way that when an external electrical voltage is applied, a mechanical reaction of the piezoelectric element ensues that, depending on the crystalline structure and the contact regions of the electrical voltage represents compression or tension in a predeterminable direction. Such piezoelectric actuators are suitable for instance for applications in which reciprocating motions take place under high actuation forces and at high cycle frequencies.
For example, one such piezoelectric actuator is known as a component of a piezoelectric injector in so-called common rail injection systems (CR injector), from German Patent Disclosure DE 100 26 005 A1. In this piezoelectric actuator, a stack of a plurality of electrically and mechanically coupled-together piezoelectric elements is present, and this stack is retained between two stops under initial tension via an actuator foot and an actuator head. Each piezoelectric layer is set between two inner electrodes, by way of which an electrical voltage can be applied from outside. Because of this electrical voltage, the piezoelectric elements then each execute short reciprocating motions in the direction of the potential drop, and these add up to make the total stroke of the piezoelectric actuator. This total stroke can be varied by way of the magnitude of the applied voltage and can be transmitted to a mechanical final control element.
In these known CR injectors, there is a nozzle needle controlled indirectly by the piezoelectric actuator; via a sleeve and diaphragm arrangement as a sheath, which compensates for stroke and temperature, the piezoelectric actuator is surrounded directly or indirectly by the pressure of the fuel, which in such low-pressure systems is approximately 60 bar.
In CR injectors with direct control of the nozzle needle, however, this known concept cannot readily be adopted, since in this case the piezoelectric actuator is operated in the high-pressure region of the fuel, and a rigid sleeve and diaphragm arrangement as a sheath would not withstand the high system pressures and the longer reciprocating motions of the piezoelectric actuator, under the space conditions available in the piezoelectric injector.
To achieve an electrical and mechanical insulation of the piezoelectric actuator, a flexible sheath of the piezoelectric actuator is often also proposed. From German Patent Disclosure DE 102 30 032 A1, to avoid the above-described disadvantages, an arrangement with a piezoelectric actuator in bathing media is known in which the piezoelectric elements are cast in an amorphous insulation compound, which in turn is placed in a housing jacket that is solidly closed off from the medium laterally and on the upper and lower ends.